Radiation converters comprising a directly converting semiconductor layer enable a counting and/or an energy-selective detection of individual quantum absorption events that are incident in the semiconductor layer by way of a detection area. In this case, the radiation quanta, for example gamma or X-ray quanta, are absorbed in the semiconductor layer and converted into free charge carriers. The liberated charge carriers are accelerated in an electric field generated by the application of a voltage between a counterelectrode and a pixelated read-out electrode. The associated charge carrier transport in the semiconductor layer induces currents on the read-out electrode, said currents being tapped off by a read-out electronic unit and detected as an electrical signal.
The conversion of radiation quanta into free charge carriers is effected by in part multistage interaction processes with a semiconductor material from which the semiconductor layer is produced. Appropriate semiconductor material includes materials having high atomic numbers in order to achieve virtually complete absorption of the radiation quanta in the case of the realizable layer thicknesses of the material. Alongside the high absorptivity, the material must furthermore have a high charge carrier mobility in order to ensure complete conversion of the charge carriers into an electrical signal.
Semiconductor materials having such properties are based, for example, on CdTe, CdZnTe, CdZnTeSe, CdZnTeSe or GaAs compounds.
Semiconductor layers composed of such materials are conventionally grown as block crystals and electrically connected to the read-out electronic unit, for example to an ASIC, after corresponding processing and electrode application by means of soldering processes.
Limits of the quantitative and of the energy-selective detection arise on account of production-dictated defect sites in the crystal lattice, for example in the form of vacancies or interstitial atoms. These are responsible for polarization effects that lead to a reduction of the charge carrier lifetime/mobility product (μτ product) and thus to an increase in the average residence duration with at the same time a reduction in the lifetime of the charge carriers in the semiconductor material. This reduces the separation efficiency of the liberated charge carriers. There is a risk, in particular, of signals from quanta that arrive in close temporal succession being superposed in such a way that it is no longer possible to unambiguously separate the events. However, liberated charge carriers can also completely recombine with oppositely charged defect sites present, such that they are completely lost for conversion into an electrical signal.
Taking this as a departure point, the intention is to provide a radiation converter comprising a directly converting semiconductor layer which makes it possible to carry out a counting and/or energy-selective detection of absorption events in an improved form. Furthermore, the intention is to specify a method for producing such a radiation converter.